it practical, allowing the earliest all-metal airliners like the Ford Trimotor designed by William Stout, and Junkers' own pioneering airliners like the Junkers F.13 to be built and accepted into service. When Andrei Tupolev likewise used the Junkers firm's techniques for all-metal aircraft construction, his designs ranged in size to the enormous, 63 meter (206 ft) wingspan eight-engined Soviet Maksim Gorki, the largest aircraft built anywhere before World War II. The de Havilland DH.88 Comet racer of 1934 was one of the first designs to incorporate all the features of the modern fast monoplane, including; stressed-skin construction, a thin, clean, low-drag cantilever wing, retractable undercarriage, landing flaps, variable-pitch propeller and enclosed cockpit. Unusually for such a highly stressed wing at that time it was still made of wood, with the thin stressed-skin design made possible by the appearance of new high-strength synthetic resin adhesives. The Comet was powered by two race-tuned but otherwise standard production de Havilland Gipsy Six engines with a combined output of 460 hp (344 kW). This compares for example on the one hand to the single 180 hp engine fitted to the Junkers CL.I all-metal monoplane of 1918 and to the 1,172 hp Rolls-Royce Merlin C development engine which powered the prototype Spitfire in 1936. In the 1930s development of the jet engine began in Germany and in England. In England Frank Whittle patented a design for a jet engine in 1930 and towards the end of the decade began developing an engine. In Germany Hans von Ohain patented his version of a jet engine in 1936 and began developing a similar engine. The two men were unaware of the other's work, and both Germany and Britain would go on to develop jet aircraft by the end of World War II. In Hungary, György Jendrassik began construction of the world's first turboprop engine. Civil aviation Many aviation firsts occurred during this period. Long-distance flights by pioneers such as Sir Charles Kingsford Smith, Alcock and Brown, Charles Lindbergh and Amy Johnson blazed a trail which new commercial airlines soon followed. Many of these new routes had few facilities such as modern runways, and this era also became the age of the great flying boats such as the German Dornier Do X, American Sikorsky S-42 and British Short Empire, which could operate from any stretch of clear, calm water. This period also saw the growth of barnstorming and